Inflatable safety restraint devices, or “airbag restraints,” are mandatory on most new vehicles. Airbag restraints commonly contain a collision sensor, an inflator, and an inflatable airbag. In the event of an accident, the collision sensor within the vehicle, such as an accelerometer, measures abnormal deceleration and triggers the inflator. The inflator is connected to the one or more airbags positioned within the vehicle. Upon receipt of the signal from the collision sensor, the inflator rapidly produces a quantity of inflation fluid or gas which fills the airbag and protects the passenger from harmful impact with the interior of the vehicle and may inhibit the passenger from being ejected from the vehicle.
One type of airbag that has received recent attention is an inflatable curtain airbag. Inflatable curtain airbags have also been developed in response to the need for passenger protection from lateral impacts with the side of a vehicle's interior. This situation might occur when another vehicle collides with the side of the car, or when the car loses control and rolls over causing the side of the car to repeatedly impact the ground.
Inflatable curtain airbags designed only to protect the occupant during lateral impacts need only remain inflated for 50 to 100 milliseconds. Inflatable curtain airbags which are also designed to provide protection during rollover-type impacts must remain inflated much longer, generally from 3 to 7 seconds.
Such airbags are designed to prevent the head of a vehicle occupant from emerging through the window opening or from colliding with a collision surface at the side of the vehicle. The shape of the curtain airbag generally conforms to the shape of the window area or side of the car adjacent the passenger's torso. Side inflatable curtains are typically positioned above the window of the vehicle. In general, these airbags are attached to the vehicle's “roof rail” and are positioned behind the vehicle's headliner.
Generally, inflatable curtain airbags designed only for lateral impacts may utilize “hot gas” inflators since cooling of the gas and loss of airbag pressure is not an issue due to the short duration of the event. However, inflatable curtain airbags designed additionally for rollover impacts have historically used “cold gas” inflators to meet the extended pressure requirements.
These inflators have a quantity of stored gas that will be channeled into the inflatable curtain during an accident. Such inflators are generally referred to as cold gas inflators because there is generally no heating of the gas as the gas is channeled into the airbag. In fact, if there is any “heating” of the gas in a cold gas inflator, such heating is insignificant and is much less than the amount of cooling that occurs when the gas expands out of the inflator.
Once the gas from a “cold gas” inflator has entered the airbag, this gas will continue to warm up (due to ambient heat) and will thus maintain a relatively high pressure within the inflatable curtain for long periods of time. As such, the inflated inflatable curtain will thus be capable of protecting the occupant throughout the duration of the rollover/crash.
Unfortunately however, such cold gas inflators are generally heavy and very large. Accordingly, these cold gas inflators can be difficult to install and costly to manufacture.
In contrast to cold gas inflators, “hot gas” inflators are those inflators that will heat the gas during deployment of the airbag. Hot gas inflators include pyrotechnic inflators which operate to produce the quantity of inflator gas via ignition of a pyrotechnic material (such as sodium azide). Another type of a hot gas inflator is the so-called “hybrid” inflator which includes both a pyrotechnic material and a quantity of a stored gas. In general, the heat that is used to warm the gas is generated, at least in part, from the ignition of the pyrotechnic material.
The heating of the gas during deployment means that a hot gas inflator uses fewer moles (molecules) of gas to provide the same volume of gas. As explained by the fundamental chemical equation PV=nRT, as the temperature of the gas increases, the volume of the gas will likewise expand. Accordingly, because the hot gas inflator provides heat to the gas, fewer moles of the gas (or gas producing material) are needed in order to fill the volume of inflatable curtain. Thus, a hot gas inflator can be smaller and lighter, and yet still provide the requisite volume of gas needed to inflate the inflatable curtain during deployment.
These smaller, lighter hot gas inflators are (1) easier to install on the vehicle and (2) cheaper to manufacture; accordingly, airbag manufacturers often, given a choice, choose to use hot gas inflators rather than cold gas inflators. Unfortunately however, hot gas inflators do have their own limitations. Specifically, after the hot gas has been channeled into the airbag, this hot gas will begin to cool back down to ambient temperature. Such cooling of the gas reduces the pressure of the airbag. Thus, the hot gas inflator does not produce an airbag that maintains a high pressure for extended periods of time. Rather, over time (as the gas cools), the airbag will begin to deflate as the gas diminishes in pressure.
Rollovers and/or other significant crashes can often last long periods of time (i.e., several seconds). Accordingly, most manufacturers want the inflated inflatable curtain to maintain a high pressure several seconds in order to properly protect the occupant during the rollover. Because exising hot gas inflators cannot maintain these high airbag pressures for long periods of time, many manufacturers have chosen not to use hot gas inflators in their inflatable curtain systems; rather, the manufacturers choose to use the larger, cold gas inflator to ensure that the pressure in the inflatable curtain remains at an optimal level for the desired period of time.
One additional problem associated with hot gas inflators is that of “heat soak out.” Specifically, because the ignition of the pyrotechnic material in the hot gas inflator produces such quantities of heat, there is generally large amounts of heat that are stored within the inflator even after the gas has been deployed into the airbag. (This heat is generally held in the filter/diffuser of the inflator). Again over time, this heat will slowly diffuse out of the inflator into the area surrounding the inflator. As these surrounding areas are often made of plastic or fabric, this “heat soak out” melts and/or destroys these plastic/fabric parts.
Accordingly, it would be an advancement in the art to provide a new type of hot gas inflator that may be used on a inflatable curtain airbag system, but will still provide extended pressure within the inflated cushion for extended periods of time. Additionally, it would be an advancement in the art to provide a new type of hot gas inflator that contains minimal residual heat and thus eliminates “heat soak out” issues with surrounding parts. Such a device is disclosed herein.